Nickel-base superalloys are used as the materials of construction of some of the components of gas turbine engines that are exposed to the most severe and demanding temperatures and environmental conditions in the engines. For example, the turbine blades and vanes, seals, and shrouds are typically formed of such nickel-base superalloys. During service, these components are exposed to temperatures of 2000° F. or more, and also to the effects of the high-velocity flow of the hot combustion gases. To perform at this high temperature for extended periods of time and many engine cycles, the materials used in the components must have good rupture strength, a sufficiently high melting point, good thermal shock resistance, and good oxidation resistance at such high temperatures.
These components are also exposed during service to hot-corrosion attack at intermediate temperatures in the range of from about 1500° F. to about 1700° F. In this temperature range, alkali metal salts such as Na2SO4 found in the combustion gas may condense on the component and produce an accelerated, severe corrosive attack. Such alkali metal salts typically result from the ingestion of sodium chloride in sea salt and its subsequent reaction with sulfur oxides during the combustion of the fuel.
The selection of the alloy compositions of the components exposed to these different types of temperature and environmental conditions poses some difficult challenges. Elemental additions and compositions that produce good high-temperature properties often lead to unsatisfactory corrosion resistance at intermediate temperatures, and vice versa. Coatings have been developed to alleviate some of the oxidation and corrosion attack, but high-aluminum coatings may lead to phase instability in the interdiffused regions during long-term exposure at the highest temperatures.
There is an ongoing need for nickel-base superalloys and articles made from such superalloys that achieve a better combination of high-temperature and intermediate-temperature properties than available superalloys. This need is particularly acute for superalloys used to make single-crystal articles, as these articles are used at the highest temperatures. The present invention fulfills this need, and further provides related advantages.